Graphene, a two-dimensional sp2-hybridized lattice of carbon atoms, has generated intense interest due to its unique electronic, mechanical, chemical, and catalytic properties. Recent synthetic efforts have focused on the development of high-yield and scalable methods of generating graphene. These techniques include the direct exfoliation of either chemically modified or pristine graphene directly into various solvents. For example, graphene oxide (GO) can be exfoliated from graphite via acidic treatments. The resulting GO flakes contain hydroxyl, epoxyl, carbonyl, and carboxyl groups along the basal plane and edges that render GO strongly hydrophilic. The ease of dispersing GO in solution has facilitated the preparation of GO thin films and GO-polymer nanocomposites with interesting and potentially useful mechanical properties. However, due to the defects and consequent disruption of the graphene band structure introduced during oxidation, GO is a poor electrical conductor. Although the level of oxygenation can be partially reversed through additional chemical reduction steps, significant quantities of structural and chemical defects remain. Moreover, the electrical conductivity of reduced GO flakes is less than optimal and is certainly deficient by comparison to pristine graphene.
In an effort to circumvent such GO limitations, recent efforts have focused on direct solution-phase exfoliation of pristine graphene. Graphene sheets can be extracted using superacids, by sonication in surfactant solutions and through use of organic solvents. For example, superacids have demonstrated an unprecedented graphene solubility of 2 mg/mL through the protonation and debundling of graphitic sheets. However, the resulting solutions are not ideally suited for additional processing due to their acidity-dependent solubility and high reactivity. Direct exfoliation of graphene in surfactant solutions and select organic solvents has also been demonstrated with concentrations up to 0.3 mg/mL and 1.2 mg/mL, respectively, but such concentrations are achieved only following prolonged sonication times—approaching 3 weeks in duration—or extended ultracentrifugation.
Concurrently, printed electronics offers an attractive alternative to conventional technologies by enabling low cost, large area, flexible devices that have the potential to enable unique advances in varied applications such as health diagnostics, energy storage, electronic displays, and food security. Among available manufacturing techniques, inkjet printing-based fabrication is a promising approach for rapid development and deployment of new material inks. The main advantages of this technology include digital and additive patterning, reduction in material waste, and compatibility with a variety of substrates with different degrees of mechanical flexibility and form-factor. Various technologically important active components have been inkjet printed including transistors, solar cells, light-emitting diodes, and sensors. Despite these device-level advances, the ability to pattern low-resistance metallic electrodes with fine resolution remains an important challenge, especially as the field evolves towards highly integrated systems.
As discussed above, graphene is a prominent contender as a metallic component in printed electronic devices due to its high conductivity, chemical stability, and intrinsic flexibility. In particular, graphene inks provide an alternative to conventional carbon-based inks that have shown limited conductivity, especially in formulations compatible with inkjet printing. However, such an application requires the production of large-area graphene that can be easily manipulated into complex device architectures. Some of the primary methods that are being explored for the mass production of graphene include growth by chemical vapor deposition (CVD), sublimation of Si from SiC, and solution-phase exfoliation of graphite or reduced graphene oxide (RGO). Among these approaches, solution-phase exfoliation offers significant advantages such as inexpensive raw materials, potential for scalability, low thermal budget, and compatibility with additive printing techniques. Exploiting these attributes, previous studies have demonstrated inkjet printing of RGO for organic thin-film transistor electrodes, temperature sensors, radio frequency applications, and chemical sensors. Nevertheless, since the electrical properties of RGO are inferior to graphene, inkjet printing of pristine graphene flakes is expected to have clear advantages in electronic applications.
Graphene can be directly exfoliated by ultrasonication in select solvents and superacids, or through the use of additives such as planar surfactants and stabilizing polymers, resulting in relatively small (<10 μm2 in area) graphene flakes. While small flakes are necessary for stable inkjet printing, they introduce an increased number of flake-to-flake junctions in percolating films, which renders them more resistive compared to CVD grown or mechanically exfoliated graphene. Moreover, traditional solvents and surfactants employed for graphene exfoliation leave persistent residues even following extensive annealing, further disrupting the conductive network.
Processing complexities represent a bottleneck for fundamental studies and end-use applications that require well-dispersed, highly concentrated, pristine graphene solutions. Accordingly, there remains an on-going search in the art for an improved approach to graphene solution concentrations—of the sort suitable for inkjet printing and related applications—sufficient to better realize the benefits and advantages available from graphene and related material compositions.